Electrically small loop antennas, that is, antennas whose total conductor length is small compared to the associated wavelength in free space, have been used as proximity tag reader antennas in identification systems, such as badge access control systems and asset tracking systems. Normally these proximity tag reader antennas have read range capabilities of approximately five centimeters to twenty centimeters extending both in front of and behind the antenna. The surface on which the card reader antenna is mounted effectively prevents unwanted reading from the back side of the antenna if the wall thickness is greater than the read range of the antenna. A problem arises, however if the read range of the antenna is greater than the thickness of the wall. In this case, an access card could be purposely or accidentally read through the wall from the back side of the antenna thereby defeating the primary purpose of the proximity card reader which is to control the access area and maintain knowledge of the location of the access card. Normally the detection ranges of the antenna have been kept low enough that reading from the back side of the antenna would be limited by the thickness of the mounting surface. For example, normally the read ranges have been reduced to approximately ten to fifteen centimeters for an average wall. If the mounting surface was thinner than the maximum read range, the system or antenna could be de-tuned or de-sensitized intentionally. This would lower the overall read range on the back side of the antenna; however, the read range has to be significantly reduced in installations where the mounting surface is thin such as on a glass wall which is usually only approximately two and a half centimeters thick. Alternatively, the system would have the normal read range but suffer from the undesirable possibility of reading an access badge from the other side of the mounting surface or wall.
Another problem arises if the proximity card reader antenna is mounted on or near a metallic surface. A metallic surface can reduce the read performance of the antenna to an unacceptable level due to the eddy current losses and the de-tuning of the antenna. This de-tuning can, in some cases, be compensated to a limited extent by re-tuning the antenna when it is installed; however, the read range of the re-tuned antenna is inevitably degraded from the original read range capability.
It can be seen that the prior art proximity access card readers have provided a less than satisfactory solution in many installations. In some instances the read range has been decreased to prevent reading from the back side of the card reader antenna. In other instances to increase the read range of the reader antenna the undesirable possibility of reading from the back side of the antenna through the surface on which it has been mounted has been tolerated. In still other installations the performance of the reader has been significantly degraded by the metallic surface on which it is mounted and has resulted in additional expense in the installation because of the need to re-tune the antenna at the installation site. The proximity readers used in asset tracking systems are troubled by the same problems as the proximity access card readers and face the additional challenge that the read range normally must be maximized to ensure detection of the tag associated with the asset.